An examination of various rhesus monkey (Macaca mulatta) organs has shown type C viral antigen expression preferentially in the placenta. Separate cocultivations of isolated primary trophoblasts from ten rhesus monkey placentas with cell lines from heterologous mammalian species led to rapid isolation of type C rhesus retroviruses in four of ten cases. These four retrovirus isolates have been designated MMC-2 through MMC-5. Distinction of these viral isolates from the initial rhesus isolate (MMC-1) and the previous isolate from the stumptail monkey, Macaca arctoides, (MAC-1) could be made by host range studies and liquid DNA hybridization, but not by limited restriction endonuclease digestion. Specifically, the cellular DNA from rhesus isolates MMC-2 through MMC-5 melted 0.7oC to 1oC lower than either MAC-1 or MMC-1. Using our hybridization conditions, it was not possible to distinguish between MAC-1 and the originally reported isolate from rhesus, MMC-1. Both MAC-1 and MMC-1 were obtained in single, long-term cocultivation experiments (over seven months). The present isolates, MMC-2 through MMC-5, were detected in two to five weeks. The prompt detection of type C particle expression following cocultivation of rhesus trophoblast cells with heterologous cell lines demonstrates that trophoblast, as a differentiated cell type, is relaxed for complete retroviral expression. It remains to be seen if other primate species, including higher apes and man, will yield infectious retrovirus using this approach.